Internet of Things
The Internet of Things (IoT) society has expectations of billions of connected sensor and actuation devices. Such devices are expected to be operational several of years on a coin-cell battery, while still being produced at extreme low implementation cost. To achieve these goals, peak communication rates are optimized for different use cases; power efficient physical layer methods as e.g. modulation techniques are utilized, as well as reusable implementation designs.
In Industrial Scientific Medical (ISM) non-licensed shared spectrum bands, many technologies share and co-exist with devices enforced to follow etiquette rules. Examples of such etiquette rules imposed by regulators are: Listen-Before-Talk (LBT) mechanisms, limits on medium utilization, and limits on transmit power. Besides various WiFi technologies, two popular technologies for IoT Machine to Machine (M2M) communication in the 2.4 GHz ISM-band are Bluetooth and Zigbee. One recent variant of the Bluetooth technology is called Bluetooth Low Energy (BLE), and its standardized physical layer (PHY) is based on Gaussian Minimum Shift Keying (GMSK) modulation. Zigbee on the other hand uses the physical layer of the Institute of Electrical and Electronics Engineers (IEEE) 802.15.4 standard and is based on Direct Sequence Spread Spectrum (DSSS) and utilizes Offset Quadrature Phase Shift Keying (O-QPSK) modulation. Ecosystems with existing chipsets HardWare (HW) supporting Bluetooth and Zigbee are huge, with e.g. Bluetooth available in 2.2 billion mobile devices.
GMSK
GMSK is a modulation method for providing phase shift keying. It is employed in many wireless and satellite communications technologies, including Global System for Mobile Communications (GSM) and Bluetooth Low Energy (BLE). A block diagram of a GMSK modulator is shown in FIG. 1. It comprises a differential encoder 110, a Gaussian low pass filter 120 and an FM modulator 130.
GMSK provides constant envelope and compact spectrum. GMSK is flexible and may be tuned to give a compromise among spectral compactness, equalization and/or demodulation complexity, and link performance. A designer of the physical layer for a wireless communication system may choose a bit period, also called symbol duration, T, a bandwidth-time product BT and a differential encoder. The bandwidth-time product determines the bandwidth of the Gaussian low pass filter 120. A small BT product will increase the spectral compactness but will also increase Inter-Symbol Interference (ISI), which in turn will require a more advanced receiver or will result in performance losses, when compared to a GMSK signal with the same bit period but larger BT. The differential encoder 110 has an impact on the trade-off between receiver performance and receiver complexity. Different differential encoder choices lead to different receiver designs, with varying degrees of complexity.
In FIG. 1, the input bits to the differential encoder 110 are denoted by ak and the output bits bk. The differential encoder 110 may be designed in many different ways. A few examples are given below, where the sum is understood to be modulo 2. That is, the bits are considered to be elements in the Galois Field GF(2).                No differential encoding: bk=ak. For example BLE does not employ a differential encoder. This type of encoder is useful when it is desired to allow low complexity differential detection at the receiver.        bk=ak+ak-1. This type of differential encoding is used in GSM.        bk=1−(ak+bk-1). This type of differential encoding is useful when two bit differential detection is used at the receiver.        
The list above is not exhaustive, and indeed there are more types of differential encoders available for a PHY designer (e.g. encoders designed to enable three bit differential decoding). The list illustrates that various differential encoders are chosen by designers of wireless systems.
O-QPSK
O-QPSK is a linear modulation technique that is known to be spectrally equivalent to Minimum Shift Keying (MSK). That is, MSK and O-QPSK modulated signals exhibit the same spectrum. A block diagram of an O-QPSK modulator is shown in FIG. 2. In this figure, the input chips ck∈{−1, 1} are transformed to the baseband time domain signal y(t). First, the chip stream is split into two streams, the first comprising the even chips and the second comprising the odd chips. An impulse train modulated by the chips are passed through a pulse shaping filter p(t) in order to obtain the baseband signal. In O-QPSK,
      p    ⁡          (      t      )        =      {                                                      sin              ⁢                                                          ⁢                                                π                  ⁢                                                                          ⁢                  t                                                  2                  ⁢                  T                                                      ,                                          when                ⁢                                                                  ⁢                0                            ≤              t              ≤                              2                ⁢                T                                                                                      0            ,            otherwise                              
Although both GMSK and O-QPSK are good modulation choices for IoT applications, devices using them are not able to understand each other. Meanwhile, since two of the most popular IoT technologies, Bluetooth and Zigbee, are based on GMSK and O-QPSK, respectively, such HW and implementations are already vastly present in the market.
Discussions are in progress within the IEEE 802.15.4 standardization to potentially also introduce a mode utilizing GMSK modulation beyond the already existing O-QPSK modulation technology. The amendment to the standard is developed in the IEEE 802.15.4t workgroup and defined for operation in the 2.4 GHz ISM-band.
In U.S. Pat. No. 7,869,409, a multi-mode transmitter which is adapted to modulate a data packet communicated by a wireless communications signal is disclosed. The data packet header is modulated with a spread spectrum technique, while the data may or may not be modulated using a spread spectrum technique. In addition, the transmitter includes an encoding circuit coupled to a so-called PN-code generator. This encoder circuit performs O-QPSK to MSK encoding, which comprises binary differential decoding followed by alternate bit inversion. Since the encoder circuit is coupled to the PN-code generator i.e. only used when spread spectrum modulation is used, the encoding circuit only applies binary differential decoding, and it is only applicable in an MSK transmitter.